Many people experience eye defects causing vertical double vision problems in some cases, and lateral double vision problems in other cases. Such problems can be caused by injury to the retina, cataract operations, and so forth.
There have been many attempts in the prior art to provide apparatus and methods for prescribing eyeglasses to correct double vision problems. It is known to use prisms in eyeglasses to correct double vision problems. A brief discussion of the teachings of known references follows below.
Edmunds, U.S. Pat. No. 439,130, teaches the use of prisms in eyeglasses for providing a magnification to the user in a binocular manner. Note beginning at line 38, it is recognized that convex lenses used by normal sighted persons may cause double vision. It is indicated that by using prism-shaped lenticular convex surfaces, correction of the double vision problem is obtained.
Pixley, U.S. Pat. No. 1,091,701 teaches an instrument for detecting heterophoria. The instrument taught is used to test the muscles of the eyes for determining the appropriate lenses for correcting double vision or diplopia. The instrument permits the muscle error to be determined in the affected eye, for prescribing an appropriate prism.
Ames, Jr., et al., U.S. Pat. No. 2,118,132 teaches the use of eyeglasses having multiple lenses for each eye, with the lenses being prismatic in their configuration, for correcting a defect in the eye known as "aniseikonia", that causes blurring of images due to differences in the size and/or shape of the images in binocular vision. It is indicated that when prisms are used to correct a condition known as "phoria", the prisms themselves may cause the user to have distortional aniseikonia.
Glazer, U.S. Pat. No. 2,442,849 teaches the use of prismatic lenses and eyeglasses for correcting double vision, particularly when users move their line of sight away from a limited central zone. It is recognized that prismatic lenses used for correcting one defect, may create other problems such as causing double vision, and eye strain.
Gernet, U.S. Pat. No. 4,427,272 shows an eyeglass lens provided with a prismatic shape, but of stepped thickness, for providing progressive magnification. The lens is associated with aniseikonia eyeglass lenses. This type of lens is used for correcting defects in binocular vision in which the two retinal images of an object differ in size.
An article by Emanuel S. Glasser, O.D., entitled "Considerations When Prescribing Large Degrees of Prism", which appeared in "Optical Journal and Review of Optometry", Jan. 1, 1965, describes the use of large degrees of prism in eyeglasses for correcting eye defects, particularly diplopia or double vision. Note in the second full paragraph of page 23, it is indicated that by properly choosing the large prism factor, the prism in combination with the patient's remaining muscle function will correct the double vision problem. It is noted that vertical correction of double vision problems is most difficult insofar as attaining the combination of both prism and muscle correction. The prism factor is apparently chosen by having a patient's head fixed in position, and requesting the patient to gaze at an object with both eyes open, while different lenses are tested, until a prism factor is found for correcting the double vision problem. As a result only a single rigid line of vision with complete correction of diplopia is obtained.